1. Field of the Invention
This invention relates to a cryocooler cooling a superconducting material using an inert gas, and more particularly relates to a heat driven acoustic orifice type pulse tube cryocooler for installing metal knit within a driving section cooling a driving gas of an application device using a principle of high temperature superconductivity, and then homogeneously heating the driving gas by way of premixed combustion.
2. Description of the Background Art
Generally, a cryocooler is applied to a field of infrared rays sensor cooling, a field of cryooperating and MRI, a field of electronic equipments such as RF filter for mobile communication, and a filed of a superconductivity electric power application device, which, for example, are driven at about 77K (−196° C.). The temperature at about 77K (−196° C.) means a cooling temperature of high temperature superconducting material using a liquified nitrogen gas. The cyrocooler cooling the superconducting material is classified as Stirling cryocooler, Joule-Thomson cryocooler, Gifford-McMahon cryocooler, and Pulse Tube cryocooler, depending upon a thermodynamic cycle.
The Gifford-McMahon cryocooler has some problems that, since each of a high temperature part and a low temperature part is provided with one or more driven part, operating efficiency is low, cooling capacity is small, and a maintenance and repair cost is very high.
FIG. 1 is a constitutional view for explaining a operation principle of the prior Stirling cryocooler. The cryocooler consists of a compression space 1a and a expansion space 1b which each volume of operating gas thereof is changed by movement of a piston 1c, a hot heat exchanger 5a, a cold heat exchanger 5b, and a regenerator 3.
The operational principle of the Stirling cryocooler will be give herein below.
To begin, if the piston 1c of the compression space 1a and the piston 1d of the expansion space 1d are moved from left to right on the basis of the drawing with constantly keeping the distance between them, an operating gas within the compression space 1a is compressed. At this time, a temperature of the operating gas rises up to TH+dTH, wherein TH is a temperature of the hot heat exchanger 5a, and dTH is a predetermined increased temperature. (S10˜S20)
If the piston 1c is moved from left to right continuously with keeping a constant pressure, a heat of the operating gas within the compressing space 1a having comparatively a higher temperature than that of the wall surface of the hot heat exchanger 5a is emitted to the outside via the hot heat exchanger 5a. (S20˜S30)
Simultaneously, the heat of the operating gas is transferred to an inner matrix of the regenerator 3 via the hot heat exchanger 5a. Then, the heat from the matrix of the regenerator 3 is transferred to the cold heat exchanger 5b, thereby the cold heat exchanger 5b has a higher temperature Tc than the prior temperature. Then, the temperature Tc of the cold heat exchanger 5b changes the temperature of the expansion space 1b. The temperature of the operating gas within the expansion space 1b into which the heat of the comparatively higher temperature is input becomes immediately higher, and the operating gas is expanded. At this time, since the piston 1d is moved according to thermal expansion of the operating gas within the expansion space 1b, the temperature within the expansion space 1b becomes Tc-dTc, wherein Tc is a temperature of the cold heat exchanger 5b, and dTc is a predetermined decreased temperature. (S30˜S40)
Meanwhile, if the piston 1c of the compression space 1a and the piston 1d of the expansion space 1d are moved from right to left on the basis of the drawing with constantly keeping the distance between them, an operating gas is compressed, and thereby the piston 1d is moved from right to left on the basis of the drawing, and simultaneously the operating gas receives heat from the outside, since the temperature of the operating gas within the expansion space 1b becomes relatively lower than that of the cold heat exchanger 5b. (S4˜S10)
Then, the operating gas within the compressing space 1a receives a predetermined heat from the hot heat exchanger 5a receiving heat from the matrix of the regenerator 3 and having a higher temperature, and has a temperature TH of the hot heat exchanger 5a. 
Here, a heat transfer quantity that the operating gas receives from the regenerator 3 during the steps S40 to S10 is equal to a heat transfer quantity that the operating gas of the compressing space transfers to the regenerator 3 during S20 to S30. Thus, a sum of the heat transfer quantity that the regenerator 3 gives and takes every cycle is numerically “0”.
Accordingly, the Stirling cryocooler carrying out a thermal dynamic cycle in regular sequence of S10, S20, S30, S40, and S10 may have a cold effect receiving heat from the low temperature part and emitting heat to the high temperature part.
However, the Stirling cryocooler has a complex structure, since each of a high temperature part and a low temperature part thereof is provided with an additional driving part. Further, the operation reliability is considerably lower due to friction between a sealing member of displacement apparatus such as a piston and a cylinder at low temperature in case of being driven for a long time
The Pulse Tube cryocooler, which is transformed from the Stirling cryocooler, is classified as a basic type, an orifice type, and a double inlet type. Further, it is classified as a resonance tube type, 2 valve-type, 4 valve-type and a mixed type according to a structure for a freezing temperature and a freezing capacity.
FIG. 2 is a constitutional view of the prior orifice type pulse tube cyrocooler. According to this type of cyrocooler, after a gas having a predetermined temperature is periodically poured into a tube having a closed end, the cyrocooler is operated according to the change of pressure of a poured gas. In case of having a little turbulent current in the gas flow, it uses a heat pumping effect enabling to obtain very high temperature gradient.
This orifice type pulse tube cyrocooler is consisted of a compressing section 2a, an aftercooler 5c subsequently connected to the compressing section 2a, a regenerator 3-1, a pulse tube 7a, a diffuser 7b, a cold gas reservoir 7d, and an orifice 7c installed between the diffuser 7b and the cold gas reservoir 7d. 
Here, the compressing section 2a is changed into a expanding section according to its motion, and it is provided with a reciprocating piston 2c within the inside. Here, it is assumed that the pulse tube 7a have a virtual gas piston.
The comparison with the orifice type pulse cryocooler and the Stirling cryocooler is as follows.
The combination structure of the pulse tube 7a, a hot heat exchanger 5a and the cold gas reservoir 7d corresponds to the expansion space 1b of the Stirling cryocooler.
Whereas the piston 1c of the compressing space 1a and the piston 1d of the expansion space 1b in the Stirling cryocooler is moved in simultaneous phase, the virtual gas piston within the pulse tube 7a of the orifice type pulse tube cryocooler is moved in the same phase as the piston 2c of the compressing section 2a by the cold gas reservoir 7d. 
Therefore, the phase difference (generated from the relationship of pressure and mass flow quantity within the pulse tube) between the piston 2c of the compressing section 2a and the virtual gas piston of the pulse tube is generated between the pulse tube 7a and the cold gas reservoir 7d. 
The phase difference generated from the orifice type pulse tube cryocooler is smaller than the phase difference generated from the piston 1c of the expansion space 1b of the Stirling cryocooler. Therefore, the cooling effect of the orifice type pulse tube cryocooler is comparatively higher. However, the orifice type pulse tube cryocooler requires more mass flow quantity per cold capacity than the Stirling cryocooler as to the amplitude of pressure change.
Meanwhile, whereas the Stirling cryocooler requires two of more driver such as the compressing space 1a and the expansion space 1b, the pulse tube cryocooler is provided with only one driver. Therefore, the orifice type pulse tube cryocooler has a more simple structure and is inexpensive for the maintenance and repair cost even an operation for a long time in comparison with the Stirling cryocooler, but still has a problem that the vibration occurs.
FIG. 3 is a constitutional view of the prior heat driven acoustic pulse tube cryocooler. This crycooler is consisted of a driving gas reservoir 9a, an electric heater 9b subsequently connected to the driving gas reservoir 9a, a cylindrical tube 9c, a heat-acoustic driver 9d, a drive stack 9e, a cold stack tube 9f, and a pulse tube 7e. Here, the other side of pulse tube 7e is provided with a diffuser 7b and a cold gas reservoir 7g. 
The operation of this cryocooler will be given herein below.
If an electrical heating source of the electric heater 9b generates a pressure pulse of a driving gas reserved in the driving gas reservoir 9a, the pressure pulse then adiabatically compresses and expands the gas. Thereafter, the temperature of the gas is changed, and then heat corresponding to the temperature of the gas is transferred to the drive stack 9e and the cold stack tube 9f. As such a result, the cryocooler is operated.
Here, the heat transferred to the pulse tube 7e is exchange in the pulse tube 7e due to a circulation of a coolant within the pulse tube 7e, and thus the heat is emitted from the pulse tube 7e to the outside.
However, this type of cyrocooler has a limited cooling capacity due to a limitation of capacity of the electrical heat source, since a very low temperature is realized by changing the electrical heat source into the acoustic energy.